Let’s talk about electric voltage loss of USB Cable

For USB cables, the most basic electrical parameters are resistance, voltage loss, voltage, etc. Of course, the most frequently asked question by customers is how much current the corresponding USB cable can withstand, and what size conductor is more cost-effective. Today we will talk about the USB cable. Basic knowledge such as resistance, voltage loss and current, today we will explain in detail about popular science, what is the cause of voltage loss? The line resistance plays a decisive role in the magnitude of the voltage loss.

Before we start talking about this charging efficiency, let's call a keyword "voltage drop". The voltage drop refers to the voltage difference between the two ends of the wire. For example, if the input end of the wire is connected to a 5V power supply, but only a voltage of 4.8V is detected at the output end, the voltage drop of the wire is 0.2V. So how does the pressure drop come about? In fact, although the material used in our USB Cable is a good electrical conductor, it is not a superconductor after all, and there is resistance inside, so when we connect the charger and the mobile phone with a USB Cable, it is equivalent to connecting in series in a circuit There is a resistor, and after the charging loop is formed, there will be current flowing through the USB Cable. With the existence of resistance and current, a voltage will naturally be generated at both ends of the wire, and the value of this voltage is the voltage drop value.

So why is voltage drop a key word for judging charging efficiency? That is because in the charging process, the input voltage of the terminal equipment is processed by the "voltage drop" of the USB Cable. For a simple example, when the output voltage of the charger is 5V and the current of the charging circuit is 2A, the voltage drop is used. A 0.2V USB Cable means that the input voltage of the terminal device is 4.8V, and the total input power is 9.6W; when a USB Cable with a voltage drop of 0.4V is used, it means that the input power of the terminal device is only 2A*4.6V =9.2W, the wire brings an additional 0.4W loss. The lower the input power, the slower the charging speed, which is the main reason why the voltage drop of the wire can affect the charging efficiency.

And the above is often just a theoretical calculation. In fact, many terminal devices have a minimum charging voltage requirement. For example, a certain device supports a charging voltage of 5V±5%, that is, 4.75V to 5.25V. When you use a device at 2A When the voltage drop under the current reaches 0.4V, the charging input voltage is likely to be only 4.6V, and the charging current has to be reduced to reduce the voltage drop, or even stop charging directly, until a USB cable with a reduced voltage can be replaced. recover. Theoretically speaking, when the voltage drop of a USB Cable reaches 0.4V under the current of 2A, if it is desired to drop to 0.25V, then the passing current must drop to 1.25A, and the input power of the terminal device is only equivalent to 4.75 V*1.25A≈5.94W, which is a significant drop compared to the original theoretical 5V*2A=10W.

How is the voltage drop caused? The line resistance plays a decisive role in the size of the voltage drop

Since the "voltage drop" has a significant impact on the charging efficiency of the USB Cable, which attribute of the USB Cable will have a significant impact on the "voltage drop"? In fact, according to the calculation formula of voltage drop "voltage = current * resistance", we can know that the resistance of the wire will have a significant impact on the voltage drop. The resistance of the wire is also what we often call "wire resistance". According to the calculation formula of "resistance = resistivity * length / cross-sectional area", we can know that when the material of the cable is the same, that is, the resistivity is the same, the wire The resistance of the cable is proportional to the length and inversely proportional to the cross-sectional area. Therefore, if you want to reduce the resistance of the cable, shortening the length and increasing the cross-sectional area is the most direct method.

This is why some relatively long USB Cable are often thicker, because it needs to increase the cross-sectional area to make up for the line resistance caused by the length, but this approach often greatly increases the cost of the wire. So these longer and thicker cables tend to be more expensive as well. However, there are also some products that do not change the cross-sectional area of the wire while increasing the length. The longer the length of such a USB cable, the more obvious the voltage drop. Of course, we do not mean that such a USB cable cannot be used, but the charging efficiency of such a wire is indeed It will be lower, and to put it simply: the resistance is the thickness of the wire [the AWG number of the conductor], the thicker the wire [the smaller the AWG number of the conductor] the greater the current, the thinner the wire [the larger the AWG number of the conductor] ], the greater the current; because: the thicker the line, the faster the current flows, the smaller the resistance, just like a water pipe, the thinner the water pipe, the smaller the water flow, the thicker the water pipe, the greater the water flow, the above popular truth.

In addition, the wire with the same length and cross-sectional area does not necessarily have the same wire resistance, and the material used for the wire is also a key factor. At present, copper wires are generally used in USB cables. Some high-end products may use silver-plated wires or even pure silver wires to reduce wire resistance, but some low-end wires are made of aluminum. The conductivity of aluminum is not bad. But it is much lower than copper. It may not have much impact on USB cables with very short lengths, such as products with a length of only 10-15cm, but for cables with a length of 1 meter, 1.5 meters or even 2 meters or more, the wire resistance caused by the aluminum material will not be affected. ignored.

Calculated with pure copper and pure aluminum, the resistivity of the latter is 1.6 times that of the former, which means that under the same length and the same cross-sectional area, the voltage drop brought by the latter will be 1.6 times that of the former. We have previously measured the charging USB cable that comes standard with Apple's MacBook Pro 16. Its wire resistance is 0.125Ω, and the voltage drop generated when passing a 4.7A current is about 0.6V, which is equivalent to a loss of 2.82W of energy. If the wire is changed from copper to aluminum, then theoretically, its wire resistance will become 0.200Ω, and the voltage drop under 4.7A current will become 0.94V, which is equivalent to a loss of 4.42W of energy.

It is worth mentioning that the "high voltage and low current" that is still in the mainstream fast charging mode was developed to solve the problem of reducing the charging efficiency caused by the voltage drop of the wire. The charger also outputs 18W power, which is equivalent to a 5V environment. At a current of 3.6A, the voltage drop is 0.36V on a wire with a wire resistance of 0.1Ω, which exceeds the requirement of ±5%, and the energy loss is as high as 1.3W; and for a 9V environment, it is 2A current, The voltage drop is only 0.2V, which is equivalent to a power loss of 0.4W, and the power loss of the latter is less than one-third of the former. Therefore, whether it is the current mainstream "high voltage and low current" mode or the gradual trend towards a unified PD charging protocol, as the charging power increases, the voltage increase is often larger than the current increase, which can not only reduce the power loss on the wire At the same time, it can also prevent the wire from becoming too thick in order to pass a large current, which is inconvenient for users to use.

Of course, the USB Cable is not only used for charging, but also for data transmission. However, compared with the line used for charging, the line used for data transmission can be said to have very low requirements on the wire, because the current passing through it is very small, and it is more often used to express the level of the level. , so when we cut a USB cable, we will see that the cable of its power supply line is obviously thicker than the cable of the USB Cable. It is precisely because of this that for the USB transmission wire, whether the connection is firm is more important than whether the wire is thick enough and the resistance is low enough. Because of this, most third-party cables now emphasize the current passing capability of their own products, and the data transmission capability is more represented by whether they support USB 3.0/3.1 or Thunderbolt 3.

Generally, the USB cable signal line is designed according to the 2.0 test standard for 28AWG transmission as the mainstream. If the test is attenuated, the longest can pass through 4 meters; the signal line part is selected, if the DC voltage drop is considered to be 125mV required by the association, the 28AWG power line is used The recommended length is within 1 meter, the recommended length of 26AWG power cord is no more than 1.7m, the recommended length of 24AWG power cord is no more than 2.7m, the recommended length of 22AWG power cord is no more than 4.3m, and the recommended length of 20AWG power cord is no more than 5m .In accordance with the USB2.0 standard test specification reference; the association recommends the following 5 specifications of conductor power cross-sectional area: